Tension measuring devices or, as they are commonly referred to in the art, line tensiometers, function to provide a continuous measure of the tension in a rope or cable or any equivalent line such as might be used in a derrick, hoist, crane or similar structure. The measured tension will provide an indication of the load or weight being lifted and it is important in such operations to monitor carefully the line tension to avoid overloading of the structure and possible breaking of the supporting line. For convenience in terminology, the cable, line or rope involved will hereafter simply be referred to as a rope, it being understood that any equivalent line, cable, chain or the like could be substituted.
Many presently available tension measuring devices or line tensiometers comprise essentially three pulleys or sheaves mounted in tandem relationship. The outer pulleys constitute first and second guide pulleys and the central pulley constitutes a force responsive element. By threading a rope under the first guide pulley, over the central pulley and thence under the second guide pulley and by so positioning the pulleys that the entrance and exit angle of the rope over the central guide pulley forms an angle less than 180.degree., it will be appreciated that a tension on the line will exert a downward force on the central pulley; that is, a force on the pulley in a direction tending to move the rope into a straight line. This force can be measured and calibrated to indicate the tension in the line by an appropriate load cell transducing means responsive to the force exerted on the central pulley.
The major problem with all tension measuring devices of the foregoing type is that the rope involved must be threaded through the device, one end of the rope first being passed under the first guide pulley, thence over the central pulley, and thence under the second guide pulley. In other words, access must be had to at least one end of the rope thereby necessitating disconnection of this end from a crane or other structure. While it is possible to design certain tension measuring devices in such a manner that the same can be attached to a rope without access to the ends, the modified designs in question involve a major alteration of the basic principle involved in measuring the force. It would be highly desirable if presently available standard tension measuring devices such as described heretofore could be modified in some simple manner without involving a great deal of expense to permit their use on ropes without requiring access to one or both ends of the rope in attaching the devices.